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beepbeep beepbeep is offline
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Join Date: Jun 2001
Location: Sweden
Posts: 5,464
I have some limited expirience with increased fan speed on aproject I worked with. We increased fan gearing on turbo-converted 3.6 N/A engine to 2.41. At 7000 RPM, fan was spinning @ 16900 RPM and severe belt problems appeared. Generator inertia was too big and it was tearing the belt apart. Fan tip speed @ 7000 RPM was aproaching M 0.66. We sourced problems to the fact that rapid RPM changes made heavy OEM generator accellerate/deccellerate too quickly which teared up the belt in zero time. Also, car wore out the generators quickly.

You can see the flat belt, belt-tensioner (actually a 944 cam belt tensioner) and "outsourced" generator situated on the right side of the fan here:


It was finally solved by using flat belt and fitting lighter generator situated out of the fan and driven by separate belt with lower gearing. I believe biggest problem with severely turbocharged aircooled engines isn't so much cylinder temperature as it is head temperature.

Also, curved 3.6 N/A fan flows less air than "930-style" straight-bladed fan. It's quieter though.

I strongly suggest checking belt wearing issues if deciding to gear up the fan. OEM V-belt is just capable of default gearing and might fail if subjected to up-geared fan.
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Last edited by beepbeep; 02-13-2007 at 07:13 AM..
Old 02-13-2007, 05:53 AM
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